MAMMALIA. 
41 
the foetus of mammalians, at an early period, has a vessel analogous to that which 
contains the yolk in the oviparous classes, receiving, in like manner, vessels from the 
mesentery. It has also another external bladder named the allantoid, which communi- 
cates with the urinary one by a canal termed the urachus. 
Conception always requires an effectual coitus, in which the fecundating fluid of the 
male is thrown into the uterus of the female. 
The young are nourished for some time after birth by a fluid peculiar to this class 
(the milk), which is produced by the mammae, at the time of parturition, and for as 
long a period as the young require it. It is from the mammae that this class derives 
its name, and, being a character peculiar to it, they distinguish it better than any 
other that is external.* 
DIVISION OF THE CLASS OF MAMMALIA INTO OEDERS. 
The variable characters which establish essential differences among the mammalia 
are taken from the organs of touch, on which depends their degree of ability or 
address, and from the organs of manducation, which determine the nature of their 
food, and are connected together, not only with all that relates to the digestive func- 
tion, but also with a multitude of other differences extending even to their intelligence. 
The degree of perfection of the organs of touch is estimated by the number and the 
mobility of the fingers, and from the greater or less extent to which their extremities 
are enveloped by the nail or the hoof. 
A hoof which envelopes all that portion of the toe which touches the ground, blunts 
its sensibility, and renders the foot incapable of seizing. 
The opposite extreme is where a nail, formed of a single lamina, covers only one 
of the faces of the extremity of the finger, and leaves the other possessed of all its 
delicacy. 
j The nature of the food is known by the grinders, to the form of which the articula- 
I tion of the jaws universally corresponds. 
I For cutting flesh, grinders are required as trenchant as a saw, and jaws fitted like 
[ scissors, which have no other motion than a vertical one. 
For bruising grain or roots, flat-crowned grinders are necessary, and jaws that 
have a lateral motion : in order that the crowns of these teeth should always be 
irregular, as in a mill, it is further requisite that their substance should be formed of 
parts of unequal hardness, so that some may wear away faster than others. 
Hoofed animals are all necessarily herbivorous, and have flat- crowned grinders, in- 
asmuch as their feet preclude the possibility of their seizing a living prey. 
Animals with unguiculated fingers are susceptible of more variety ; their food is of 
all kinds : and, independently of the form of their grinders, they differ greatly from 
each other in the mobility and delicacy of their fingers. There is one character with 
respect to this, which has immense influence on their dexterity, and greatly multiplies 
its powers ; it is the faculty of opposing the thumb to the other fingers for the purpose 
of seizing small objects, constituting what is properly termed a hand; a faculty which 
* We shall find, however, in the sequel some doubts on this sub- i to be no nipples, simple pressure alone causing the fluid to exude, 
ject, as regards the family of Monotremata. [These doubts have In the class of birds, a lacteal fluid is secreted by the crops of the 
since been removed, inasmuch as the lacteal glands have been de parrots and pigeons, which is disgorged into the throats of the young 
tected, with their secretion ; though, as in the cetaceans, there appear I w'heu newly hatched. — En.] 
